You're pretty close, but it's actually going to be a part of Enchanting. Once every 24 hours, through Enchanting (not Alchemy) you'll be able to split a Void Crystal into two Large Prismatic Shards. This is through a new recipe that's being added in 2.4.

This helps reduce the exorbitant prices currently being seen for Large Prismatics, and helps raise the value of the Void Crystals. While this is the only change planned in 2.4 to help the Void Crystal prices, we're still looking at the possibility of additional 'help' for Void Crystal value in the future.